For e.g to disable the add to cart button in product detail page you can add below code.
Go to app\design\frontend\\\template\catalog\product\view\addtocart.phtml
Add below code after getProduct();?>
<?php if($_product->getTypeId() != 'simple'): ?>
<?php endif; ?>
so your file look like below.
<?php $_product = $this->getProduct();?>
<?php if($_product->getTypeId() != 'simple'): ?>
<?php $buttonTitle = $this->__('Add to Cart'); ?>
<?php if($_product->isSaleable()): ?>
<div class="add-to-cart">
<?php if(!$_product->isGrouped()): ?>
<label for="qty"><?php echo $this->__('Qty:') ?></label>
<input type="text" name="qty" id="qty" maxlength="12" value="<?php echo $this->getProductDefaultQty() * 1 ?>" title="<?php echo $this->__('Qty') ?>" class="input-text qty" />
<?php endif; ?>
<button type="button" title="<?php echo $buttonTitle ?>" class="button btn-cart" onclick="productAddToCartForm.submit(this)"><span><span><?php echo $buttonTitle ?></span></span></button>
<?php echo $this->getChildHtml('', true, true) ?>
</div>
<?php endif; ?>
<?php endif; ?>
And you are done!